1. Field of the Invention
The present invention relates to electrophotographic image forming apparatuses, more particularly to an image forming apparatus in which photoreceptor drums are electrically charged by charging units using proximity discharge.
2. Description of Related Art
Recently, to achieve a low cost per print (CPP), image forming apparatuses have been researched and developed so as to have a longer service life. Such research and development activities often target a longer service life of photoreceptor drums. For example, some photoreceptor drums have protective layers provided on their surfaces. The protective layer renders the surface of the photoreceptor drum less wearable and less polishable.
Furthermore, the electrophotographic image forming apparatus might have an image defect called image deletion. Image deletion is said to occur when the surface of the photoreceptor drum is rendered less resistant due to adhesion of a discharge product, so that latent image charges flow in the sub-scanning direction of the photoreceptor drum.
To prevent image deletion, the image forming apparatus under the circumstances in which image deletion is likely to occur (e.g., a high-temperature, high-humidity environment) executes a recovery mode to remove a portion of the photoreceptor drum surface that is less resistant (hereinafter, such a portion will be simply referred to as a “low-resistance portion) (see Japanese Patent Laid-Open Publication Nos. 2004-212623 and 2011-209490).
In Japanese Patent Laid-Open Publication No. 2004-212623, the recovery mode changes the output of an alternating-current component of the voltage applied to a charging unit (e.g., a charging roller) using proximity discharge. As a result, the photoreceptor drum is rendered more wearable, so that the low-resistance portion can be removed.
In Japanese Patent Laid-Open Publication No. 2011-209490, prior to cleaning of the photoreceptor drum, the current flowing through a charging unit (e.g., a charging roller) using proximity discharge is measured when a measurement voltage is applied to the charging unit. The amount of toner to be supplied during cleaning is set on the basis of the measurement result. Thereafter, development is performed with the amount of toner being set, and the toner supported on the surface of the photoreceptor drum is carried to a cleaning blade. As a result, the abrasive performance of the blade is enhanced, so that the low-resistance portion is removed.
However, the photoreceptor drum surface with a protective layer provided thereon has a problem in that it is difficult to remove the low-resistance portion from the surface even if the recovery mode is executed.